Magnetic recording media generally comprise at least one magnetizable layer (also commonly referred to as an "information storing layer" or "magnetic recording layer") coated onto at least one side of a substrate. For particulate magnetic recording media, the magnetizable layer comprises a magnetic pigment dispersed in a polymeric binder. The polymeric binder of a magnetic recording medium is most commonly prepared from a polymer blend comprising a hard component, i.e., a polymer with relatively high glass transition temperature and modulus, and a soft component, i.e. a polymer with relatively low glass transition temperature and modulus. In addition to the binder and magnetic pigment, the magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
Some forms of magnetic recording media, such as magnetic recording tape, may also have a backside coating applied to the other side of the substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating also includes a polymeric binder and other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
The polymeric binders of the magnetic layer and the backside coating are commonly derived from polymers which require curing in order to provide magnetic recording media with appropriate physical and electromagnetic properties. To prepare such media, the components of the magnetic layer or the backside coating, as appropriate, are combined with a suitable solvent and thoroughly mixed to form a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the wet coating is passed through a magnetic field in order to orient, or randomize in some cases, the magnetic pigment. The oriented coating is then dried, calendered if desired, and then cured.
One problem associated with the manufacture of magnetic recording media relates to the strength of the magnetic field that can be used to orient the pigment in the wet magnetic layer after coating. Generally, it is desirable to use as strong of an orientation field as possible in order to enhance the electromagnetic properties of the resultant magnetic recording layer. Yet, if the magnetic field is too strong, the field can cause undesirable movement of the magnetic pigment. Such field induced movement can lead to orientation roughness and/or migration of the magnetic pigment in the dispersion. It would be desirable to find a technique that would allow higher orientation fields to be used while avoiding the problems of orientation roughness and pigment migration.
Another problem associated with manufacturing magnetic recording media relates to the tendency of magnetic pigments to agglomerate. Because of such tendency, magnetic pigments can be difficult to initially disperse in the polymeric binder and/or can be difficult to keep dispersed in the polymeric binder over time. Preferably, therefore, the magnetizable layer incorporates ingredients having characteristics which promote good wetting and dispersion of the magnetic pigment and reduces the tendency of the pigments to agglomerate.
Traditionally, good pigment wetting has been achieved by selecting suitable low molecular weight dispersing agents which interact strongly with the pigment surface and provide stability to the magnetic particles once dispersed. For higher pigment loadings, i.e., the use of greater amounts by weight of magnetic pigment, greater amounts of wetting agent or dispersant may be required. This is not always desirable. Dispersants tend to plasticize binder systems and decrease their modulus. Further, excess dispersant may exude from a cured binder system over time, leading to changes in the properties of the media as well as to contamination of a recording head or the like.
To help alleviate the problems associated with added low molecular weight dispersants or wetting agents, polymeric binders formed from "self-wetting" polymers have been developed. "Self-wetting" polymers have dispersing groups pendant from the polymer backbone that help disperse the magnetic pigment. Representative examples of dispersing groups include quaternary ammonium, amine, heterocyclic moieties, salts or acids based on sulfate, salts or acids based on sulfonate, salts or acids based on phosphate, salts or acids based on phosphonate, salts or acids based on carboxyl, mixtures thereof, and the like. As a result of using self-wetting polymers, less low molecular weight dispersant or wetting agent, or even no low molecular weight dispersant or wetting agent, may be needed to disperse the magnetic and nonmagnetic (if any) pigments in the polymeric binder.
For example, U.S. Pat. No. 5,510,187 describes one example of a so-called "self-wetting" polymeric binder system incorporating, in preferred embodiments, a quaternary functional, nonhalogenated vinyl copolymer in combination with a self-wetting polyurethane polymer. This kind of binder system has proved very effective in achieving good dispersions of magnetic particles, allowing magnetic recording media with excellent mechanical and electromagnetic properties to be prepared. Although the outstanding performance of this binder system is most likely due to many factors, it is believed that the quaternary ammonium functionality of the vinyl copolymer is a very important contributor to the success of this binder system.
Notwithstanding the excellent performance of this kind of binder system, advances in magnetic pigment technology demand even further wetting and modification of the pigment surface in order to achieve more uniform dispersion of the magnetic pigment. Previously, techniques for treating the surfaces of magnetic pigments with surface treatment agents in a manner that eases dispersion and reduces agglomeration have been proposed. For example, according to this approach, surfaces of a magnetic pigment may be modified so that the surface treated pigment becomes more compatible with the binder.
Indeed, a wide variety of surface treatment agents and methods are known. However, due to the performance capabilities of the kind of binder system described in U.S. Pat. No. 5,510,187, it would be extremely desirable to have a surface treatment approach for magnetic pigments that is particularly suitable for use in combination with binder polymers having quaternary ammonium functionality, particularly quaternary ammonium functional, nonhalogenated vinyl copolymers.